Abstract

Porous nickel oxide (NiO) and zinc oxide (ZnO) submicro- and nanofibers were synthesized
by impregnating electrospun polyacrylonitrile (PAN) fiber templates with corresponding
metal nitrate aqueous solutions and subsequent calcination. The diameter of the NiO
and ZnO fibers was closely related to that of the template fibers and larger diameters
were obtained when using the template fibers with larger diameter. SEM results showed
that the NiO and ZnO fibers have a large amount of pores with diameters ranging from
5 nm to 20 nm and 50 nm to 100 nm, respectively. Energy dispersive X-ray (EDX) spectra
and X-ray diffraction (XRD) patterns testified that the obtained materials were NiO
and ZnO with high purity.

Keywords:

Nickel oxide; Zinc oxide; Nanofiber; Porous material; Template

Introduction

Porous materials with high specific surface area (SSA) are highly required for applications
in catalysis, supercapacitors, electromechanical actuators, and bio- and environmental
engineering [1-4]. Nanomaterials in the dimension range of 1 to 100 nm have received considerable interest
because of the unique properties different from their bulk counterparts. Porous nanomaterials
have attracted much attention in recent years due to their hierarchical nanostructures
[5-9].

In this work, we developed a novel approach to synthesize porous NiO and ZnO submicro-
and nanofibers by impregnating electrospun polyacrylonitrile (PAN) fiber templates
with corresponding nitrates aqueous solution and subsequent calcination. This method
is very simple and possibly applicable for preparing porous submicro- and nanofibers
of other metal oxide materials.

Experimental

Polyacrylonitrile (PAN,Mw = 150,000) purchased from Aldrich was used to prepare the fiber templates. The electrospinning
solutions were prepared by dissolving PAN in dimethylformamide (DMF) solvent at the
mass concentration of 11 and 14%. The electrospinning process was carried out at the
voltage of 20 kV, where aluminum foils were used as the collectors. The collected
PAN fibers were pretreated at 250 °C for 2 h in air for stabilization. About 50 mg
stabilized PAN fiber templates were impregnated with 10 mL Ni(NO3)2or Zn(NO3)2aqueous solution at desired concentrations. Then the samples were impregnated with
10 mL 0.1 M ammonia solution. After drying in air, the samples were heated from room
temperature to 700 °C in air at a heating rate of 10 K/min and maintained for 1 h
in air.

X-ray diffraction (XRD) pattern was recorded with a XD–2 diffractometer (Beijing Purkinje
General Instrument Co., Ltd.) to identify the phase of the samples. The morphological
features of the samples were characterized with a HITACHI S-4700 scanning electron
microscopy (SEM), and their components were determined by an energy dispersion X-ray
spectroscope (EDX) (EDAX company) equipped in the SEM system.

Results and Discussion

Figure 1 shows the SEM images of the electrospun PAN fibers prepared at the solution concentrations
of 11 and 14%. The diameter of the PAN fibers is strongly dependant on the solution
concentration with it in the range of 310–410 nm (Fig. 1a) at the former concentration and 850–1050 nm (Fig. 1b) at the latter concentration. The diameter increase of the electrospun fibers with
solution concentration is due to the resultant increase in solution viscosity, which
has been repeatedly reported previously [25,26].

Figure 2gives the SEM images of the NiO (a–c) and ZnO (d–f) products at different magnifications.
Both products exhibit fibrous morphology. The diameter of the NiO and ZnO fibers ranges
from 80 to 130 nm and 400 to 750 nm, respectively. The template fibers used for preparing
the NiO and ZnO fibers were electrospun at the solution concentrations of 11 and 14%,
respectively. As the above stated, the PAN fibers electrospun at the concentration
of 11% is much smaller than that electrospun at 14% (Fig. 1). So the diameter difference of the NiO and ZnO fibers originates from that of their
respective template fibers. It is observed that the NiO nanofibers are composed of
many nanoparticles with diameter in the range of 20 to 35 nm, and the ZnO submicrofibers
are composed of many nanocrystallites with a width of about 65 nm and length of about
200 nm. Interestingly, both the NiO and ZnO fibers exhibit porous morphology with
pore size in the range of 20 to 35 nm for the former and 50 to 100 nm for the later
(Fig. 2).

Besides the diameter of the PAN template fibers, the concentration of the impregnating
solutions (Ni(NO3)2and Zn(NO3)2) is also important in affecting the diameter of the product fibers. Different concentrations
of 0.6, 1.0, and 3.0% for the nickel nitrate solutions were investigated for preparing
the NiO fibers, where the PAN fibers prepared at the PAN/DMF solution concentration
of 11% were used as the templates. The SEM images of these three samples are shown
in Figs. 3a,2a–c, and3b, respectively. With decreasing the concentration of the nickel nitrate solution
from 3.0% to 0.6%, the diameter of the NiO nanofibers decreases from about 210 to
about 80 nm, while the pores in the NiO nanofibers show no obvious changes.

Figure 4 shows the typical EDX spectra of the NiO and ZnO fibers. For the spectrum of the
NiO fibers only nickel and oxygen were observed (Fig. 4a) and for the ZnO fibers only zinc and oxygen were observed (Fig. 4b). Furthermore, the atomic ratios of nickel/oxygen in the NiO fibers and zinc/oxygen
in the ZnO fibers are near to the theoretical ratio of NiO and ZnO of 1:1. The XRD
patterns of the NiO and ZnO fibers are presented in Fig. 5a and b, respectively. The well-defined diffraction peaks at about 37.6, 43.7, 63.4,
75.7, and 79.8° are observed in Fig. 5a, corresponding to the (111), (200), (220), (311), and (222) planes of cubic NiO
crystals [14], respectively. The peak positions of the XRD pattern of the ZnO fibers (Fig. 5b) are in good agreement with those of the hexagonal ZnO with lattice constants a = 0.3250 nm and c = 0.5207 nm [27]. The above EDX and XRD results testify that the obtained products are NiO and ZnO
with high purity, respectively.

Many literatures reported that the products with tubular structures could be obtained
when calcinating the polymer fiber templates coated with a shell layer of inorganic
precursors [28,29]. However, some researchers also pointed out that the shell layer of the inorganic
precursors would shrink or collapse if the process of removing the templates could
not be well controlled [30]. The phenomenon of shrinkage of the shell layers without forming tubular structures
provides us a simple and easy approach to fabricate fibrous materials. The reason
for the formation of the NiO and ZnO fibers rather than tubes mainly lies in that
the precursor NiO and ZnO shell layers coated on the surface of the PAN template fibers
shrink accompanying with the removal of the PAN fibers maybe driven by external pressure
and the cohesion between the nanoparticles. To promote the shrinkage of the shell
layers, low concentration of nitrate precursor solution and high temperature heating
rate were adopted. The adoption of low concentration of precursor solution results
in the small thickness of the coating layer and thus low self-supporting strength.
In this case the shell layers are prone to shrink under external pressure during removal
of the core templates. The adoption of high heating rate of 10 K/min results in rapid
oxidation of the PAN fiber templates and release of a large amount of gases during
short time, which generates certain pressure difference between the exterior and interior
of the fibers. If this pressure difference is higher than the critical pressure beyond
which the shell loses its stability, the shell layer would shrink [31], and thus products with solid structure are obtained.

The PAN fiber templates were removed through decomposition, carbonization, and oxidation
due to reaction with O2 in air [32], which generated a large amount of gas products. The release of these gases from
the core of the impregnated fibers to the outside could produce a large number of
pores in the residual materials [33,34]. Although using the same amount of nickel nitrate and zinc nitrate precursors, the
NiO fibers have much smaller diameter than the ZnO fibers (Fig. 2), strongly suggesting that the structure of the ZnO fibers is less compact. This
is in agreement with the observation that the pore size of ZnO fibers is larger. Furthermore,
the more porous structure of ZnO fibers results in their poorer mechanical strength,
and thus the ZnO fibers are prone to break. Therefore, it is vital to carefully control
the procedure of removing the polymer fiber templates for fabricating long, continuous,
and porous metal oxide submicro- and nanofibers.

Conclusion

In conclusion, porous NiO and ZnO submicro- and nanofibers were prepared by using
electrospun polyacrylonitrile fibers as templates. The products were obtained by impregnating
the electrospun fibrous webs with nickel and zinc nitrate aqueous solution followed
by calcination in air. The diameter of the NiO and ZnO fibers is strongly dependant
on that of the template fibers and tends to be larger when using the templates with
larger diameter. SEM showed that the NiO and ZnO fibers have many pores with diameter
in the ranges of 5 to 20 nm and 50 to 100 nm, respectively. The EDX and XRD results
testified that the obtained products are NiO and ZnO with high purity. We expect that
the procedure of removing the polymer template fibers play key roles in the structure
formation of the products. This method is very simple and possibly applicable for
preparing porous submicro- and nanofibers of other oxide materials.

Acknowledgments

This work was supported by the NSFC (Grant No. 50572019), Program for New Century
Excellent Talents in University (NCET), SRF for ROCS, SEM, and S&T Program of Shenzhen
government.